1. Field of the Invention
The present invention relates to a dual-phase steel sheet excellent in bake hardening property capable of ensuring high strength by applying coating and baking (hardening property after bake coating, hereinafter sometimes referred to as BH (Bake Hardening) property) and a stretch flange formability. Specifically, this invention relates to a high strength dual-phase steel sheet excellent in the bake hardening property described above and having a low yield ratio, as well as excellent in the balance for strength-elongation and the balance for strength-stretch flange formability.
2. Description of Related Art
Steel sheets used by pressing in the industrial fields such as for automobiles, electric apparatus and machineries are required to have both excellent strength and ductility together and such demands for the characteristics have been increased more and more in recent years.
As steel sheets intended for making the strength and the ductility compatible, ferrite-martensite dual-phase steel sheets comprising a ferritic structure as a matrix phase in which coarse island-like martensite is dispersed at the triple point of the ferrite (dual-phase (DP) steel sheet) have been known so far (for example in JP-A No. 122821/1980).
It has been known that the DP steel sheet described above is not only excellent in ductility but also excellent in bake hardening property (BH property). In the DP steel sheet, a great amount of C solid solubilized at super saturation in ferrite (solid solution C) are present since the sheet is manufactured by quenching from a temperature of A1 point or higher. It is considered that the yield strength of the steel sheet is increased and the BH property is enhanced by fixing of solid solubilized C to dislocations of the ferrite introduced during working by the bake coating step after the working. However, since the amount of solid solution C that can be present at super saturation in the ferrite is limited, it was difficult to obtain BH property above a certain level.
Further, while the DP steel sheet has high tensile strength (TS) at low yield ratio and also has excellent elongation (EI) property, since coarse martensite induces trigger for fracture, it was poor in the stretch flange formability (local ductility: λ).
Then, in order to improve the stretch flange formability in the DP steel sheet, the present applicant has already disclosed a tri-phase steel sheet comprising ferrite, bainite, and martensite [Tri-Phase (TP) steel sheet] (JP-A No. 39770/1983). In the steel sheet described above, since martensite inducing fracture is surrounded by the bainite phase, the stretch flange formability is improved compared with existent DP steel sheets. However, it has been found that the steel sheet involves problems that it is difficult to obtain a high ductility (high elongation) at a level identical with that of the existent DP steel sheet and the yield ratio is increased somewhat.
Accordingly, it has been strongly demanded for the provision of a high strength dual-phase steel plate capable of maintaining (i) low yield ratio and (ii) favorable strength-elongation balance and, in addition, further intending to improve (iii) BH property [(i) low yield ratio, (ii) favorable strength-elongation balance and (iii) high BH property is the features of the DP steel sheets], as well as capable of overcoming (iv) low stretch flange formability as the drawback of the existent DP steel plate and also excellent in the stretch flange formability.